Method for operating a double-fed asynchronous machine

ABSTRACT

In a method for operating a double-fed asynchronous machine, an exciter winding of the rotor is excited by adjusting an amplitude or a frequency of a voltage or current independently of armature values of the stator to attain a predetermined phase position and amplitude in the stator. While the stator is disconnected from an electrical grid, the rotation speed of the rotor is increased during startup and the amplitude of the voltage and/or of the current flow is adjusted to less than a start-up limit value, while the frequency of the voltage and/or of the current flow is adjusted to a grid frequency. The winding arrangement is then connected to the electrical grid, and the amplitude of the voltage and/or of the current flow is adjusted to an operating value which is greater than the start-up limit value by a predetermined amount.

The invention relates to a method for operating a double-fedasynchronous machine.

A double-fed asynchronous machine has a stator and a rotor. The statorcomprises a winding arrangement that is preferably connected, duringoperation of the asynchronous machine, to an electrical grid, forexample the 50 Hz interconnected grid. The rotor has an exciter winding,which is connected to a current converter, in particular an inverter.Via the inverter the exciter winding can be connected indirectly to theelectrical grid. Via the current converter, depending on the operatingmode of the asynchronous machine, electrical energy can be fed from thegrid into the rotor or from the rotor into the grid. In this case thespeed of rotation of the rotor can lie below the synchronous speed(subsynchronous operation) or above the synchronous speed(supersynchronous operation).

The asynchronous machine is excited via the exciter winding. In thiscase the electrical power taken up or emitted by the stator (activepower and reactive power) is regulated via the excitation of theasynchronous machine or of the rotor. To excite the rotor or theasynchronous machine an electrical voltage is applied via the currentconverter to the exciter winding and/or an electrical current isinjected into the exciter winding. To do this the current converter hasa control unit, which measures the speed and angle via a rotary positiontransducer and amplitude and phase position of the voltage and/or of thecurrent in the winding arrangement via electrical measurementconverters. In other words the excitation of the rotor will be regulateddepending on the speed and the angle of the rotor and also depending onthe amplitude and phase position of the voltage and/or of the current inthe winding arrangement.

Moreover regulation methods without rotary position transducers areknown, in which the angle of the rotor is determined via the amplitudesand phase positions of the currents and voltages in the stator and inthe rotor. In summary this always involves a closed-loop regulationcircuit for regulating the rotor voltage of the rotor current.

EP 2 001 120 A2 deals with a control device for a double-fedasynchronous generator. The control device has an inverter circuit witha number of levels, which control the double-fed asynchronous generatorthrough failure modes and can recognize faulty island modes. Theinverter circuit is supplied with electrical energy on the grid side.

EP 2 200 169 A2 describes the starting up of an asynchronous machinewhile said machine is disconnected from an energy supply grid. When aspeed of for example 350 revolutions per minute is reached theasynchronous machine is demagnetized and thereafter the rotor current isswitched off. This ensures that a short-circuit switch can be openedalmost without any current. The asynchronous machine is then magnetizedagain and synchronized with the energy supply. The asynchronous machineis henceforth operated in its normal operation. When the short-circuitswitch is designed as a power switch or as a load isolator a priordemagnetization is not required.

The disadvantage of the prior art is that the converter, in particularthe inverter, must be specifically designed for the asynchronousmachine.

The object of the present invention is to make possible an improvedcontrol of the excitation of the asynchronous machine.

Not part of the claimed invention is a double-fed asynchronous machinewith

-   -   a rotor that has an exciter winding,    -   a stator that comprises a winding arrangement,    -   an electrical exciter unit for exciting the asynchronous machine        via the exciter winding, and    -   a control unit for controlling the exciter unit by adjusting at        least one electrical variable for the excitation.

To make easier control of the excitation of the asynchronous machinepossible there is provision for the control unit not to have anysignaling or electrical connection to the stator that goes beyond purelysupplying energy to the control unit, so that the at least oneelectrical variable is able to be adjusted by the control unitindependently of electrical stator variables of the winding arrangement.

For example the exciter unit involves a current source or a voltagesource. The winding arrangement of the stator is in particular athree-phase arrangement. Then, when the asynchronous machine isoperating, a three-phase alternating current flows through the windingarrangement as the stator current. The stator current refers to theoverall current that flows through the winding arrangement. A statorvoltage refers to a voltage that is applied to the winding arrangement.In particular this involves a three-phase alternating voltage. Thestator voltage, the stator current and its phase position, for examplein relation to one another and/or related to an electrical grid, areexamples of stator variables.

The exciter winding is in particular a three-phase arrangement. Theexciter unit can accordingly be configured to inject a current flow(rotor current) into the exciter winding and/or to apply a voltage(rotor voltage) to the exciter winding. This enables the excitation ofthe asynchronous machine to be achieved. The exciter unit can beembodied, depending on the operating point of the asynchronous machine,to feed electrical power into the exciter winding or to take it from theexciter winding. The control unit can be configured to influence or tocontrol this current flow and/or this voltage by adjusting the at leastone electrical variable. In other words the control unit can beconfigured to control the excitation of the asynchronous machine byspecifying the electrical variable, wherein the rotor current and/or therotor voltage are influenced or controlled in their turn by means of theelectrical variable. The at least one electrical variable relates toamplitude and/or frequency of the rotor current and/or of the rotorvoltage for example.

The current flow or the voltage in the exciter winding can be adjustedby the control unit in such a way that a pre-specified stator current isproduced in the winding arrangement in relation to amplitude and/orphase position. In other words the control unit can be embodied tocontrol the excitation so that a predetermined amplitude and/or phaseposition is produced for the stator current. Since the control unit doesnot have a signaling or electrical connection to the stator, there is nofeedback here. Values related to the rotor, for example the rotorvoltage and/or the rotor current, are able to be adjusted by the controlunit by adjusting the at least one electrical variable independently ofthe stator or without interaction with the stator. For example thecontrol unit is configured to adjust a pre-specified value for the atleast one electrical variable. Through this form of control ofexcitation of the asynchronous machine demands on the exciter unit andalso the measuring units are especially low.

The control unit and the exciter unit can be able to be connected to anelectrical grid, in particular the 50 Hz interconnected grid. In thiscase the exciter unit can be configured, on injection of the rotorcurrent or on application of the rotor voltage to the exciter winding,to feed electrical energy into the electrical grid from the exciterwinding or to feed electrical energy from the electrical grid into theexciter winding. The stator or the winding arrangement can be able to bedirectly connected to the electrical grid.

One form of embodiment makes provision for the control unit and theexciter unit to be able to be connected to the electrical grid forexclusively supplying them with electrical energy and for the windingarrangement likewise to be able to be connected to the electrical grid,wherein the control unit and the exciter unit are able to be connectedto the stator exclusively in this way for signaling or electrically. Inother words, when the asynchronous machine is operating on theelectrical grid, the exciter unit and the control unit are connected tothe stator or to the winding arrangement exclusively via the electricalgrid. Instead the control unit can be embodied to predetermine theelectrical variable or the rotor current and/or the rotor voltage atleast partly based on a frequency of the electrical grid.

The exciter unit is configured in particular to connect the exciterwinding to the electrical grid. In this case the connection betweenexciter winding and electrical grid can be switchable by the exciterunit. For example the exciter unit has at least one switching elementfor through-connecting a power supply voltage from the electrical gridto the exciter winding. The at least one switching element can becontrolled or switched by the control unit. The rotor current or therotor voltage is able to be controlled in the exciter winding bysuitable switching of the at least one switching element.

The control unit is embodied in particular to control the exciter unitin accordance with an open regulation circuit. In other words, becauseof the absence of a connection between the control unit and the stator,there is no feedback on the basis of stator variables. In particular thecontrol unit is not embodied to compare the stator variables with arequired value. On the contrary the control unit is preferably embodiedto adjust or to control the electrical variable or the rotor currentand/or the rotor voltage free from the stator variables.

One development makes provision for the control unit to comprise amemory unit, in which a characteristic field for adjusting the at leastone electrical variable can be stored, and for the control unit to beembodied to control the exciter unit on the basis of the characteristicfield. For example the at least one electrical variable for one or moreoperating points of the asynchronous machine is permanentlypredetermined by the characteristic field. In this case the control unitcan be embodied to adjust the exciter unit by adjusting the at least oneelectrical variable to at least one predetermined characteristic fieldvalue that is read out of the characteristic field. Through thecharacteristic field a suitable excitation can be predetermined for manyoperating points. In particular a comprehensive control of theexcitation is possible through the characteristic field without the needfor feedback. The characteristic field can be established or recorded ina test mode of the asynchronous machine for example.

One development makes provision for a rotation angle transducer or arotation speed transducer to be mounted on a shaft of the asynchronousmachine. This shaft can be connected mechanically to the rotor. Inparticular the shaft is arranged on the rotor and rotates uniformly withthe latter. The control unit can be configured to use its signal or itssignals to deduce the angle and/or the direction of rotation andfrequency of the stator rotating field from the angle of rotation and/orthe direction of rotation and speed of rotation of the rotor togetherwith the angle and/or the direction of rotation and frequency of therotor rotating field. This enables the frequency of the electrical gridthat feeds the stator to be determined and monitored without measuringstator variables directly.

If the frequency of the electrical grid that feeds the stator is knownfrom another source, e.g. during operation on the 50 Hz interconnectedgrid or through a control or regulation of the electrical gridindependent of the apparatus described here, it is further possible tocompare the frequency of the electrical grid with the difference betweenthe speed and the frequency of the rotor rotating field and thus tomonitor that the stator rotating field and the rotor rotating field arerunning synchronously (synchronism) and the asynchronous machine is notpulling out of step. In particular in this case the known frequency ofthe electrical grid is able to be predetermined to the control unit. Thecontrol unit can be configured to carry out the aforementionedcomparison and/or monitoring. In this case too it is not necessary forthe stator current or the stator voltage to be measured by the controlunit.

One development makes provision for the exciter unit to be designed as acurrent converter. In particular the exciter unit can be designed as aninverter. The current converter can be embodied to adjust phaseposition, frequency and/or amplitude of the rotor current in relation tothe grid voltage. In other words the current converter can adjust thegrid voltage in such a way that phase, frequency and/or amplitude of therotor current and/or of the rotor voltage correspond to the at least oneelectrical variable adjusted by the control unit. The current converteris a simple and effective option to excite the rotor controllably viathe electrical grid.

One development makes provision for the exciter unit to be embodieduniversally for providing or shaping electrical energy independently ofthe asynchronous machine. In other words the exciter unit is preferablynot embodied specifically for the purpose of exciting the asynchronousmachine, but has a universal application, for example as a currentconverter or inverter. In particular the option of using a universallyapplicable current converter as the exciter unit produces an especiallysimple structure of the asynchronous machine.

The asynchronous machine can be connectable to a load or to a drive. Forexample a shaft of the asynchronous machine, which is part of the rotor,can have a coupling element for coupling the shaft to the load or to thedrive. The load involves a mechanical load for example, which is able tobe driven by the asynchronous machine, or a further electrical machine,a so-called load machine. In particular the load is able to be operatedby the asynchronous machine in a motor mode of the asynchronous machine.The asynchronous machine can be embodied in its motor mode to transmit atorque to the load. The drive for example involves an electricalmachine, a turbine, in a power plant for example, or a wind turbine of awind turbine system. The asynchronous machine can be embodied to receivea torque from the drive. The asynchronous machine is able to be set intorotation by the drive. In particular the asynchronous machine is able tobe driven by the drive in a generator mode of the asynchronous machine.

A torsional vibration damper can be provided on the shaft of theasynchronous machine. The torsional vibration damper can contribute apre-specified portion to the moment of inertia of the rotor. For examplethe torsional vibration damper has a portion of 10% of the moment ofinertia of the rotor %. The torsional vibration damper in particularinvolves an additional moment of inertia supported sprung on the shaft,Vibrations of the shaft can be minimized through the damping of thespring element.

As an alternative or in addition a damping can be achieved for exampleby a fan wheel being mounted on the shaft.

The torsional vibration damper, the fan wheel and the load, inparticular the load machine, represent examples of how vibrations of theasynchronous machine can be reduced during operation. By connecting theasynchronous machine to the load, it is possible to reduce or compensatefor vibrations that arise through the structure of the exciter unit andthe control unit described. Likewise these vibrations can be reduced orcompensated for through the use of the torsional vibration damper. In aconventional asynchronous machine, of which the excitation is able to beregulated by feedback in a closed loop regulation circuit, such measuresare not necessary.

Claimed as the invention within the framework of the present applicationis a method for operating a double-fed asynchronous machine. The methodis based on the following steps:

-   -   excitation of an exciter winding of a rotor of the asynchronous        machine by an exciter unit, and    -   control of the exciter unit by adjusting at least one electrical        variable for the excitation.

There is provision that during the control, the at least one electricalvariable is adjusted independently of armature variables of a windingarrangement of a stator of the asynchronous machine. In other words theelectrical variable can be adjusted free of feedback. In this case theelectrical variable is adjusted solely with reference to rotor-relatedvariables for example.

The inventive method is suitable for operating a double-fed asynchronousmachine of the type described here. The inventive method and theasynchronous machine thus relate to one another. For this reasonfeatures of the inventive method also develop the asynchronous machineand vice versa. Therefore the features already described in the contextof the asynchronous machine are not described once again.

In the inventive method there is provision for the electrical variablefor the excitation to be adjusted in accordance with an open regulationcircuit. In other words the excitation of the asynchronous machine or ofthe rotor can be controlled in accordance with the open regulationcircuit. This means that a feedback on the basis of the stator variablesis dispensed with. In particular the electrical variable ispredetermined with reference to a characteristic field of the.

Moreover there is provision for amplitude and frequency of a voltage(rotor voltage) or of a current flow (rotor current) to be adjusted asthe at least one electrical variable for the excitation, so that apredetermined phase position and a predetermined amplitude is achievedin the stator. In particular a predetermined phase position and apredetermined amplitude of the stator current or of the stator voltageare achieved by this in the winding arrangement of the stator. In thiscase the phase position and/or the amplitude of the stator current iscalculated for example, since the feedback of the stator variables, i.e.stator current or stator voltage, is dispensed with for example. Forexample values for the amplitude and/or the frequency of the rotorvoltage are stored in the characteristic field, for which thepredetermined phase position and the predetermined amplitude of thestator current or of the stator voltage is produced. In this way theexcitation of the rotor can be controlled in an especially simple way.

During a startup process of the asynchronous machine there is provisionfor the chosen amplitude of the voltage (stator voltage) and/or of thecurrent flow (rotor current) to be less than a pre-specified start-uplimit value and the for the frequency of the voltage (rotor voltage)and/or of the current flow (rotor current) to be able to be adjusted toa grid frequency of an electrical grid to which the winding arrangementof the stator is able to be connected. During the startup process and/orin the operation of the asynchronous machine the stator or the windingarrangement of the stator is in particular connected to the electricalgrid, Preferably the exciter winding is moreover connected to theelectrical grid via the exciter unit. The exciter unit preferablyinvolves a current converter, especially an inverter. Thus for examplethe grid frequency of the electrical grid can be measured at the rotoror at the exciter unit. The startup limit value can be defined as afixed value, by the characteristic field for example. As part of thestart-up process the winding arrangement of the stator can subsequentlybe connected to the electrical grid or switched on. In this case thephase position of rotor and stator are adjusted to one anotherautomatically. When the winding arrangement is connected to theelectrical grid compensating currents can flow. Moreover a torque can begenerated at the rotor, through which the phase position or rotor andstator can be adjusted to one another by adjusting the rotor. The factthat the amplitude of the rotor voltage and/or of the rotor current ischosen as a smaller value that the start-up limit value enables thecompensation currents to be kept lower than a pre-specified limit value.This is needed in particular since, as a result of the absence offeedback or as a result of the open control (in accordance with the openregulation circuit), a unique phase position of rotor and stator is notable to be predetermined or adjusted. Through the start-up processdescribed however the phase position automatically adjusts itself to anongoing value.

After the connection or switching-on of the winding arrangement to/atthe electrical grid there is provision for the amplitude of the voltageand/or of the current flow to be adjusted to a predetermined operatingvalue. The pre-specified operating value in this case is in particularlarger by a pre-specified amount than the start-up limit value. Theoperating value can be adapted in this case to a rated power or to atorque of the asynchronous machine, Using this as a starting point, thepre-specified start-up limit value can be predetermined to be smaller bya pre-specified amount.

The start-up process can also occur when the asynchronous machine isalready rotating. This is the case for example when the asynchronousmachine is operated as a generator and is brought up to a specific speedby the drive machine. So that the compensation currents and the torquesare as low as possible during the compensation process, the adjustmentand frequency of the rotor rotating field can be predetermined so thatthe rotational frequency of the rotating field is produced together withspeed of rotation of the rotor at approximately the rotation frequencyof the stator rotating field. The rotor rotational field can move inthis case in the direction of rotation of the rotor and also against thedirection of rotation of the rotor.

Further features and advantages are to be found in the description givenbelow of the enclosed figures. In the figures the same referencecharacters refer to the same features and functions. The exemplaryembodiments merely serve to explain the invention and are not intendedto restrict ft.

In the figures:

FIG. 1 shows a block diagram of a double-fed asynchronous machine, whichis connected to an electrical grid;

FIG. 2 shows the course of electrical variables related to the rotor andto the stator during a start-up process of the asynchronous machine;

FIG. 3 shows the course of further electrical variables related to therotor and to the stator during a start-up process of the asynchronousmachine;

FIG. 4 shows a block diagram of a possible arrangement of theasynchronous machine; and

FIG. 5 shows a further block diagram of a possible arrangement of theasynchronous machine.

FIG. 1 shows a block diagram of a double-fed asynchronous machine 1,which comprises a stator 2, a rotor 3, an exciter unit 4 and also acontrol unit 5. An exciter winding 30 of the rotor 3 is able to beconnected to an electrical grid 6 via the exciter unit 4. A windingarrangement 20 (only shown extremely schematically in the figure) isable to be connected to the electrical grid 6 via a switching unit 21.In this case the winding arrangement 20 is designed in particular as athree-phase arrangement, for which reason a connection has three phaselegs. The switching unit 21 can be a part of the asynchronous machine 1.The electrical connection 22 between the stator 2 and the electricalgrid 6 is able to be switched by means of the switching unit 21. Inparticular the electrical connection 22 is able to be disconnected bythe switching unit 21.

The rotor 3 or the asynchronous machine 1 is able to be excited via theexciter winding 30 of the rotor 3. In this case the excitation occursvia the exciter unit 4. The excitation is controlled by the control unit5, which predetermines at least one electrical variable for theexcitation. The control unit 5 can comprise a memory unit 50, in whichpre-specified criteria for adjusting the at least one electricalvariable are able to be stored. For example a characteristic field forthe at least one electrical variable is stored in the memory unit 50.

The at least one electrical variable, which is adjusted for theexcitation, relates in particular to a frequency and an amplitude of acurrent flow (rotor current) or of a voltage (rotor voltage) in theexciter winding 30. In other words preferably at least two electricalvariables are adjusted for the excitation. By adjusting the at least oneelectrical variable the excitation of the asynchronous machine 1 or ofthe rotor 3 can be controlled.

The exciter unit 4 in the present case is designed as a currentconverter, in particular as an inverter. Via the exciter unit 4, therotor current and/or the rotor voltage of the exciter winding 30 arecontrolled and regulated in such a way that the rotor current or therotor voltage correspond to the at least one electrical variable that isadjusted by the control unit 5. For example the exciter unit 4 has oneor more switching elements that are controlled by the control unit 5.The switching elements in particular involve transistors, preferablyfield effect transistors, or IGBTs (bipolar transistor with insulatedgate electrode).

Depending on the operating mode of the asynchronous machine 1,electrical power can be fed by the exciter winding 30 into theelectrical grid 6 or by the electrical grid 6 into the exciter winding30 via the exciter unit 4.

The excitation of the asynchronous machine 1 or of the rotor 3 iscontrolled by the control unit 5 completely independently of electricalstator variables of the stator 2. Examples of stator variables arestator currents or stator voltages for example. Stator currents andstator voltages are for example individual phase currents of theindividual phases of the winding arrangement 20 as well as an overallcurrent or an overall voltage resulting therefrom. Further examples ofstator variables are the speed of rotation of the rotor 3 in relation tothe stator 2 and also an angle of the rotor 3 in relation to the stator2, This involves variables that are measured by a rotary positiontransducer of the stator in the prior art for example.

The excitation of the rotor 3 or the at least one electrical variable iscontrolled here in the sense of an open regulation circuit. Thereforeinteraction between the stator 2 and the exciter unit 4 as well as thecontrol unit 5 can be dispensed with. Different values for the at leastone physical variable can be predetermined in the characteristic fieldfor a number of operating points or operating states of the asynchronousmachine 1. The rotor voltage or the rotor current is thus controlledindependently of the stator-related variables.

Because of the absence of interaction with the stator 2 the exciter unit4 can be designed as a universally applicable current converter. Inparticular no special adaptation of the exciter unit 4 to theasynchronous machine 1 is necessary. The excitation of the asynchronousmachine 1 is adapted exclusively here by the control unit 5.

In particular the stator 2 or the winding arrangement 20, in normaloperation of the asynchronous machine 1, is connected directly, meaningin an unregulated way, to the electrical grid 6. This means that theswitching unit 21 establishes a direct electrical connection 22 of thephase legs to the electrical grid 6. If the electrical grid involves thegeneral 50 Hz interconnected grid, then the grid voltage of theelectrical grid 6 is fixed and cannot be changed by the operation of theasynchronous machine 1 as electric motor or as generator. Therefore inthis case the electrical grid 6 involves a voltage source. For this caseit has proved advantageous to control the excitation of the rotor 3 withguided current. In other words the at least one electrical variable forthe rotor current is predetermined. The exciter unit 4 represents acurrent source in this case.

In general it is possible for both the winding arrangement 20 and alsothe exciter winding 30 to each be connected to a current source or to avoltage source. It has proved advantageous however for the windingarrangement 20 to be connected to a current source and the exciterwinding 30 to a voltage source or conversely for the winding arrangement20 to be connected to a voltage source and the exciter winding 30 to acurrent source, Since the stator 2 is preferably connected statically tothe electrical grid 6, which often represents a voltage source, it hasproved advantageous to designed the exciter unit 4 as a current source.

Advantageously the rotor 3 or a shaft, which is part of the rotor 3 oris connected to the rotor 3, has a torsional vibration damper. Thetorsional vibration damper has a moment of inertia, which for exampleamounts to 10% of the rotor 3. The moment of inertia of the torsionalvibration damper can be supported sprung on the shaft. In particular themoment of inertia of the torsional vibration damper is linked to theshaft in an elastic and damped manner. As an alternative or in additiona fan wheel can be arranged on the shaft.

FIG. 2 shows a number of stator-related and rotor-related electricalvariables represented on a time scale t (in seconds). FIG. 2 shows thefollowing: The rotor current 10, the rotor voltage 11, the statorcurrent 12 and the stator voltage 13. In this case the four variablesare each shown in volts (v). In this figure the asynchronous machine isat a standstill at a time t of 0 s. Thus FIG. 2 shows a start-up processof the asynchronous machine 1. The start-up process of the asynchronousmachine 1 is especially important in the present method for controllingthe excitation, since the fields of stator 2 and rotor 3 initiallyusually do not coincide. This means that the phase position betweenstator 2 and rotor 3 is initially undefined. It is first necessary toalign the fields of stator 2 and rotor 3 in relation to one another.

To do this, in accordance with FIG. 2, the rotor 3 is first operatedwith a rotor current 10 of which the amplitude is less than apre-specified start-up limit value. The start-up limit value is less bya pre-specified amount than a pre-specified operating value to which theamplitude of the rotor current 10 is adjusted in a normal mode ofoperation, for example at rated power, of the asynchronous machine 1. Byinjecting the rotor current 10 into the exciter winding 30, a rotorvoltage 11 is produced at the exciter winding 30. The frequency of therotor current 10 is fixed at the grid frequency of the electrical grid6. The stator 2 or the winding arrangement 20 is initially stilldisconnected from the electrical grid 6.

The stator 2 or the winding arrangement 20 is connected to theelectrical grid 6. This takes place for example by closing respectiveswitches of the switch unit 21.

This produces a compensation process, in which flow in the windingarrangement currents 20 and generate a torque at the rotor 3, so thatthe rotor 3 aligns in the field of the stator 2. Through this the phaseposition between the stator 2 and the rotor 3 is adjusted appropriately.In particular the chosen pre-specified limit value is sufficiently smallfor the compensation currents not to exceed a pre-specified amount.

With the asynchronous machine 1 it is possible to change the speed ofrotation. In other words the asynchronous machine 1 offers the option ofregulating the speed. This is done in particular through a continuouschange of the frequencies of stator 2 or of rotor 3. In particular thefrequencies of the stator current and/or of the stator voltage or thefrequencies of the rotor current and/or of the rotor voltage are changedcontinuously in order to change the speed of rotation. By continuouslychanging the frequencies, the synchronism between stator 2 and rotor 3is not lost. As well as the frequency of the rotor current and/or rotorvoltage (rotational frequency), the direction of rotation of the rotorrotary field can also be changed, so that any given speed of rotation ofthe rotor can be reached. As an alternative or in addition the amplitudeof the rotor current or of the rotor voltage can be changed, in order tocontrol the active power and also the reactive power of the asynchronousmachine 1. For example, by predetermining a pre-specified value for theamplitude of the rotor current or of the rotor voltage, the active powerand/or the reactive power of the asynchronous machine can be adjusted toa further pre-specified value.

FIG. 3 shows the same start-up process of the asynchronous machine 1,wherein other values are plotted on the same time axis t (in seconds:Electrical reactive power of the rotor 40 (in kVA), electrical reactivepower of the stator 41 (in kVA), electrical active power of the rotor 42(in kW), electrical active power of the stator 43 (in kW), mechanicalpower of the asynchronous machine (in kW) and also speed of rotation ofthe rotor 46 (in revolutions per minute, rpm).

FIG. 4 and FIG. 5 each show an arrangement with the asynchronous machine1, These figures can involve test rigs for the asynchronous machine 1.For example the characteristic field for the asynchronous machine can becreated on the test rig.

In FIG. 4 the asynchronous machine 1 is shown in a test environment oron a test rig. Here the asynchronous machine 1 is operating in generatormode, in the present example the asynchronous machine 1 is drivenmechanically via two load machines 64. Electrical energy is fed from therotor 3 of the asynchronous machine 1 into the electrical grid 6,Moreover electrical energy is fed from the stator 2 of the asynchronousmachine 1 into a synchronous machine 60, The two load machines 64 arecontrolled by respective inverters 63. The inverters 63 in their turnare supplied with an electrical voltage via voltage-regulated inverters61, 62. The exciter unit 4 is embodied in this example as acurrent-regulated inverter, Electrical and mechanical energy flows areto be taken from FIG. 4.

FIG. 5 shows the asynchronous machine 1 in another test setup. Here toothe exciter unit 4 is embodied as a current-regulated inverter. Theelectrical power from the stator 2 of the asynchronous machine 1 is feddirectly to the current-regulated inverter 61 here. In the example ofFIG. 5, the test setup has only one load machine 64, which is controlledvia the inverter 63,

What is claimed:
 1. (canceled)
 2. A method for operating a double-fedasynchronous machine, comprising: exciting an exciter winding of a rotorof the asynchronous machine by adjusting at least one electricalvariable independently of armature values of a winding arrangement of astator of the asynchronous machine so as to attain a predetermined phaseposition and a predetermined amplitude in the stator, wherein the atleast one electrical variable comprises an amplitude and/or a frequencyof a voltage or of a current flow of the rotor; during a start-upprocess of the asynchronous machine and while the exciter winding isadjusted and while a winding arrangement of the stator is disconnectedfrom an electrical grid; increasing a speed of rotation of the rotor,adjusting the amplitude of the voltage and/or of the current flow toless than a predetermined start-up limit value, and adjusting thefrequency of the voltage and/or of the current flow to a grid frequencyof the electrical grid; thereafter connecting the winding arrangement tothe electrical grid; and adjusting the amplitude of the voltage and/orof the current flow to a predetermined operating value which is greaterthan the predetermined start-up limit value by at least a predeterminedamount.